1. Field of the Invention
The present invention relates to a display control unit that can appropriately display both the screen output of a computer image and a video image (e.g., image conforming to NTSC/PAL on TV screen), and to a control method therefor; and in particular to a display control unit that overlays a computer image plane with a video image plane, and to a control method therefor. More specifically, the present invention pertains to a display control unit that can overlay a computer image plane with a video image plane while not causing flickering on the screen and losing the details of a video image, and to a control method therefor. In other words, the present invention relates to a display control unit that can display image data optimally even when non-interlaced image data are converted into interlaced image data, and to a control method therefor.
2. Discussion of the Prior Art
A display device displays an image by scanning image data onto a screen. The screen scan system is roughly divided into non-interlaced scanning and interlaced scanning.
The interlaced scanning is a method for drawing over only one of an odd numbered line field and an even numbered line field during one vertical scan period (i.e., scanning every other line) so that one screen plane is completed by performing two vertical scans. Non-interlaced scanning is a method that does not involve scanning every other line, but instead provides all the scan lines to be drawn at one time so that the drawing of one screen plane is completed during one vertical scan period. FIGS. 3(a) and (b) are schematic diagrams showing the interlaced scanning and the non-interlaced scanning that are performed on the screen. Commonly, there are 525 horizontal scan lines on one screen (480 valid scan lines except for those in a blanking interval) and vertical scanning is performed once every 1/60 second, i.e., at a speed of 60 Hz. Since non-interlaced scanning draws all the scan lines each time, the vertical positions of the drawn scan lines are identical on any screen. But as interlaced scanning draws alternately over an ODD screen, which consists of 262.5 (=525/2) odd numbered scan lines, and an EVEN screen, which consists of 262.5 even numbered scan lines, for each vertical scanning, the positions of the drawn scan lines in the vertical direction are shifted by one scan line between ODD screens and EVEN screens. If, in the interlaced scanning, image data carried by the adjacent scan lines differ greatly, flickering tends to occur between the EVEN screen and the ODD screen. Especially as the size of the display device becomes larger as with a large screen TV, the flickering is more annoying.
Although a non-interlaced display device and its controller are more expensive than those for interlaced scanning, they provide excellent image quality and little flickering on the screen, and reduce eyestrain. On the other hand, the interlaced scanning can reduce by half the quantity of data to be transferred and a display device employing the interlaced scanning is inexpensive. However, as is described above, flickering occurs on the interlaced screen and its image quality is degraded. Currently, computer display devices mainly employ non-interlaced scanning. HDTVs (High-Definition Television sets), that are characterized by a clear video image and high quality sound production (i.e., realism), also employ non-interlaced scanning. On the other hand, the standards for color television broadcasting (so-called NTSC system) that is prescribed by the NTSC (National Television System Committee) still employ interlaced scanning, because an interlaced display device is inexpensive and most of the home television sets that are already in wide spread use employ interlaced scanning.
There has appeared a need, for the purpose of presentations in large conference rooms or other purposes, for displaying the non-interlaced screen of personal computers on home television sets with large screens by the NTSC/PAL format (i.e., interlaced scanning at 60 Hz or 50 Hz). When a computer image is displayed, non-interpolated, on a home television set (especially a large-screen television set), however, flickering of the image tends to occur on the screen, as compared with video images produced by a television set, a video camera, and a VTR. This is because a computer image is mainly constituted by lines and shade patterns and the difference in image data is great between adjacent scan lines.
To overcome such a shortcoming that occurs when a computer image is displayed by interlaced scanning, conventionally, an interpolation process is performed in which image data is averaged between sequential scan lines. FIG. 4 is a specific diagram illustrating a circuit for averaging image data between scan lines. In FIG. 4, a line buffer 1 is employed to temporarily store image data sequentially and to output it after a predetermined delay time has elapsed. In order to average the image data between sequential scan lines, the line buffer 1 must have a large size enough to store image data for at least a single scan line. For example, when there are 640 pixels per line, to accumulate image data with the YUV 4:2:2: format (Y is a luminance component and U/V is a color difference component), which is a common digital video format, the line buffer 1 must have a size of at least 16 bits.times.640, i.e., a size of about 1 Kbyte. An averaging circuit 2 averages two input signals. That is, the averaging circuit 2 acquires average value from the image data of pixels on a current scan line and that of the corresponding pixels on the previous scan line stored in the line buffer 1, and sequentially outputs the average value to a display section (e.g., a TV). Then the display section actually performs the drawing. The method for interpolating the image data between the scan lines is very effective in reducing the flickering between scan lines, even though the method degrades the vertical resolution. The degradation of the vertical resolution can be solved by a cooperative operation involving software (i.e., by scaling up displayed characters in computer image).
However, current multi-media type personal computers frequently overlay a normal computer image that consists of lines and shade patterns with a video image, such as motion picture and still picture provided by CD-ROM or HDD. Unlike computer images, it is not necessary to perform interpolation (see FIG. 4) for video images, as details that are characteristic of video images will be lost and the image quality will be deteriorated. Since there is only a little difference between adjacent scan lines in the image data of video image (i.e., the image information of video image is smoothly changed) unlike computer image, almost no effect is obtained by using interpolation to reduce flickering. On the contrary, the reproduction of an original image may be lost due to deterioration of the vertical resolution. However, if interpolation is not performed, the flickering in the computer image plane cannot be removed.
If interpolation is performed either manually or automatically when the total displayed plane is a video image plane, the above described shortcomings can be easily resolved. But, methods that uniformly handle the total plane can not optimize the screen including overlaid video image plane. This is because a video image plane, which is provided by current multi-media type personal computers may have unspecified size and overlie unspecified position among a computer image plane.
In addition, the necessary of the line buffer 1 that has a large size of about 1 Kbyte is a relatively great load for the design and the manufacture of an electronic circuit. A computer image itself originally does not have delicate changes in luminance and color on the screen. Therefore, as to a computer image, even if an information quantity per pixel for the line buffer 1 is reduced by throwing away the lower bits, the quality of the image is little deteriorated. In the computer graphics field, it is well known that a line buffer can be scaled down by omitting the lower bits. However, if the lower bits of a video image are also thrown away, a troublesome condition, such as the loss of the details, inevitably occurs. It is because delicate luminance changes and fine color changes are important for video image. For example, the omission of the lower bits of a Y (luminance) signal causes flickering on the screen; and the omission of the lower bits of a U/V (color difference) signal causes a change in the color phases. That is, the reduction in flickering caused by decreasing the size of the line buffer 1 has a trade-off relationship with the deterioration of the image quality.